Displacement system for controlled infusion of a liquid

ABSTRACT

In a displacement system for controlled infusion of a liquid from a cartridge comprising a tubular vessel (1), which is at a rear end closed by a piston (3) which may be forced by a piston rod (8) moving into the vessel (1) in the axial direction thereof to press out the liquid through an outlet (2) arranged at a front end of the vessel (1), the piston rod (8) is provided as a flexible incompressible construction which is by a piston rod guide (9) behind the rear end of the cartridge deflected away from the axis of this cartridge, preferably 180°. The piston rod guide (9) has a guiding track comprising a curved part and linear parts at each end of the curved part ensuring that the piston rod is guided along a length (a) in the direction of the axis of the cartridge which length (a) is longer than the distance (b) between the two axes of the piston rod parts projecting from the piston rod guide. The guiding track is further elaborated to the very shape which the curved part of the piston rod will spontaneously adopt when its end portions are kept parallel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT/DK94/00361 filed Sep. 27,1994, which is incorporated herein by reference.

The invention relates to displacement systems for controlled infusion ofa liquid from a cartridge of the kind comprising a tubular vessel, whichis at a rear end closed by a piston which may be forced by a piston rodinto the tube to press out the liquid through an outlet arranged at afront end of the tube.

Designated as pumps such displacement systems are used for therapeuticinfusion of medicine, e.g insulin for diabetics to whom the naturalproduction of insulin is simulated in this way.

A commonly used pump structure comprises a housing with a cavityaccommodating an infusion syringe cartridge having a piston and a pistonrod by which the piston may be forced into the cartridge to press outthe medicament in the cartridge through an outlet at the end of thecartridge opposite the piston. Such infusion pumps, which are based onthe technique known from syringes by which the dosage can be adjustedwith very close accuracy, has many advantages. They are simple in theirconstruction and a precise indication of the medicine left in thecartridge may currently be obtained by monitoring the position of thepiston in the cartridge, and in the same way the amount infused can bevery precise controlled by controlling the distance of movement of thepiston in the cartridge.

A heavy drawback by the pumps of the cartridge and piston type is thefact that at least one of the dimensions of the pump have to be at leastmore than two times the overall stroke of the piston in the cartridge asthe cartridge has to be at least a little longer than this stroke and asspace must be reserved for the piston rod behind the cartridge when thepiston in a new cartridge is in its rearmost position. Thereby a limitis set for the extent of the miniaturization which is aimed at to makeit as comfortable as possible to carry the pump during the activities ofa day.

Consequently it is an object of the invention to provide a displacementsystem of the cartridge/piston type wherein this drawback is avoided.

This is obtained by a displacement system of the kind mentioned in theingress of this specification and which system is according to theinvention characterized in that the piston rod is provided as a flexibleincompressible construction and at a position behind the rear end of thecartridge is deflected away from the axis of the cartridge.

The deflection of the piston rod away from the axis of the cartridge maybe obtained by a piston rod guide being provided behind the rear end ofthe cartridge. The piston rod guide governs the deflection of the pistonrod so that this rod will only be deflected in the way determined by theguide.

Preferably the piston rod is deflected 180° so that it forms twoparallel straight portions extending from the piston rod guide.

To ensure that the necessary deflecting forces on the piston rod areexerted by the guide alone so that the straight portions projecting fromthe guide are parallel without any need for deflecting forces on theseprojecting portion, the piston rod guide has a guide track having anupper curved part and a lover linear part at each end of the curved partand having a length in the direction of the projecting parallelportions, which length taken from the ends of the guide track from whichthese portions projects to the centre of the piston rod at the top ofthe curved part is equal to or larger than the distance between the axesof the projecting parallel portions.

It is also important that the piston rod is abutting the curved guidingsurface all the way so that the spring effect of the piston rod does notcause any slack in the guiding as such a slack may result in animprecise dose. This continuous abutment is obtained by elaborating thecurved part of the guide in accordance with the shape which the curvedpart of the piston rod will spontaneously adopt when its end portionsare kept parallel.

The piston rod may be made flexible in several ways. By being a flexibleband stiffened by having an arcuate cross section the piston rod has onepreferred mode of deflection as it will tend to deflect towards theconcave side of the arcuate cross section. A special stiff piston rodmay be obtained when two of the mentioned bands are combined with theirconvex sides abutting each other over the distance between the pistonand the point of deflexion. When the piston rod is deflected, the twobands are deflected in different directions away from each other as eachband is deflected toward the concave side of its cross section. Thepiston rod guide must be accordingly designed to ensure this splittingof the flexible piston rod.

Alternatively the piston rod may be a flexible helix with narrowlyadjacent turns of windings. This piston rod has no preferred mode ofdeflection and may be guided in any direction by the piston rod guide.

From SE 449 776 is known a device in which a piston is moved bytransmitting a pulling or pressing force through a flexible helix.During the working function of the device the provided flexible pistonrod is used for exerting a pulling force on a piston to very slowly sucka fluid, preferably air, into a cylinder ampoule to obtain a samplerepresentative of the ambient air over a time. The piston rod mayfurther be used for pressing the sample out of the cylinder ampoule. Itis mentioned that the wire used and the helix must be so dimensionedthat the straight portion of the piston rod does not bend out whentransmitting a pressing force through this portion and so that thewindings are not pulled apart to leave lasting deformation of the helixwhen a pulling force is transmitted.

When the piston rod according to the invention is a flexible helix, caremust be taken to ensure that the helix is so dimensioned that it maytransmit an axial pressing force without being compressed and withoutbending out as any bending out means imprecise dosage. This is avoidedby keeping the coiling ratio, i.e. the ratio between the outer diameterof the helix minus the diameter of the wire and the diameter of the wirefrom which the helix is wound, within certain limits.

Also the initial tension, which keeps the windings of the helix abuttingeach other even when a pulling force is transmitted by the helix, is ofimportance to the compressibility of the helix. The larger the initialtension the smaller the tendency for one winding to slip on the adjacentwinding to start a bending out. Consequently the initial tension shouldbe maximised.

As the pulling of the piston rod only serves the retraction of thepiston rod to its initial position when a new cylinder ampoule isinserted, no heavy pulling force is transmitted and the demands to theprecise transmission are low, but the transmission of pressing forceshave to be very precise and no compression or bending out of the pistonrod can be tolerated.

These problems are all overcome by a piston rod wound as a helix with acoiling ratio ##EQU1## where d_(helix) is the outer diameter of thewound helix, and d_(wire) is the diameter of the wire.

Preferably r_(coil) is kept <4.5, more preferably <4.0, and mostpreferably <3.5.

Further the piston rod may be a series of interconnected chain links.Such a rod may have a preferred mode of deflection or may be freelydeflected in any direction, all depending on the construction and theinterconnection of the chain links.

When the piston rod is a flexible band this band may be cogged along oneor both of its sides to provide a rack which may be engaged by a drivingpinion to move the piston rod and consequently the piston into thevessel to press out an amount of the medicine in the cartridge.

When the piston rod is provided as a narrowly wound helix the windingsof this helix may present an external thread, which may be engaged by aninternally threaded nut element which will drive the piston rod into thecartridge when this nut element is rotated and is not axial displaceablein the housing accommodating the cartridge, the piston rod guide, and adrive mechanism.

A piston rod constructed from interconnected chain links may be drivenby either of the mentioned ways as each chain link may be provided byeither a part of an external thread or with teeth making it a part of arack.

The driving force is preferably transmitted to the piston rod at alinear part thereof as the pitch of the rack or the thread are onlyunambiguously defined at such linear parts.

Preferably the driving force is transmitted to the piston rodimmediately behind the cartridge at a position between the cartridge andthe piston rod guide. Thereby transmission through the non-linear partof the piston rod is avoided.

Alternatively a driving force may applied by an advancing mechanism atthe free end of the piston rod exerting a pressing force in the axialdirection of this free end. Thereby inaccuracies caused by possiblecompressions in the piston rod are eliminated.

In the following the invention is described in further details withreferences to the drawing, wherein

FIG. 1 schematically shows a known displacement system,

FIG. 2 schematically shows a displacement system having a deflectablepiston rod.

FIG. 1 shows schematically a displacement system of the conventionalknown kind. A cartridge 1 is at one end closed by a closure enabling themounting of a catheter 2 communicating with a liquid medicine in thecartridge. At its other end the cartridge is closed by a piston 3 whichby a piston rod 4 may be pressed into the cartridge to press out theliquid medicine through the catheter 2. The pressure advancing thepiston 3 in the cartridge 1 is transmitted to the end of the piston rodthrough a presser foot 5 which is advanced by a threaded drive spindleby having a bore with an internal thread engaging the thread of thedrive spindle 6, which spindle as shown by the arcuate arrow 7 isrotated by a not shown drive mechanism controlling the rate at which themedicine is pressed out through the catheter. The Cartridge 1 and thedrive spindle 6 are mounted in a housing, not shown, so that thecartridge 1 and drive spindle 6 may not be displaced relative to eachother. The presser foot 5 may abut the end of the piston rod 4 or it maybe secured to this end.

A displacement system according to the invention is shown in FIG. 2.This system comprises to some extent the same elements as do the systemshown in FIG. 1 and these elements are numbered as the correspondingelements in FIG. 1. However, a flexible piston rod 8 replaces theconventional stiff piston 4 rod of FIG. 1. Further a piston rod guide 9is provided which guide deflects the piston rod 8 immediately outsidethe open end of the cartridge 1.

In the embodiment shown in FIG. 2 the piston rod 8 is deflected 180° sothat the outer end of this flexible rod 8 runs parallel with thecartridge whereby the overall length of the device may be reduced tocorrespond to about the length of the cartridge and the deflectingpiston rod guide 9.

The piston rod guide 9 is equipped with a guiding track conforming theouter contour of the bended or deflected piston rod 8 so that thedeflection is guided and no bending of the rod 8 is possible except forthe deflection defined by the guide 9.

The guiding track is guiding the piston rod over a length "a" which inthe axial direction of the parallel ends of the rod 8 is as long as orlonger than the distance "b" between the two axes of the parallelstraight portions of the piston rod 8 projecting from the piston rodguide 9. Each end of the guiding track are linear along a distance "d"whereby it is ensured that no deflecting forces have to be exerted onthe piston rod 8 outside the piston rod guide 9.

In the embodiment shown in FIG. 2 the driving force is exerted on theouter end of the deflected piston rod 8. The force is exerted in theaxial direction of said outer end and due to the rod 8 beingincompressible and the guide guiding the rod conforming with the profileof the flexible rod, the piston rod is displaced along its own axisround through the bending provided by the guide 9 to drive the piston 3into the cartridge 1. The drive mechanism comprising a presser foot 5acting on the free end of the piston rod 8 and a threaded spindle 6engaging an internal thread in a bore in the presser foot 5 may be ofany known type providing a rotation in the direction indicated by thearcuate arrow 7.

The flexible piston rod may be advanced by other mechanisms and thedriving force may be transmitted to the flexible rod anywhere along itslength without deviating from the scope of the invention.

We claim:
 1. A displacement system for controlled infusion of a liquidfrom a cartridge, said system comprising:a tubular vessel having a frontend and a rear end, the rear end being closed by a piston which ismovable by a piston rod moving into the vessel in the axial directionthereof to press out the liquid through an outlet arranged at the frontend of the vessel, the piston rod being a flexible incompressibleconstruction which at a position behind the rear end of the tubularvessel is deflected away from the axis of the cartridge by a piston rodguide provided behind the rear end of the cartridge and deflects thepiston rod 180° so that parallel portions of the piston rod projectsfrom one side of the piston rod guide, the piston rod guide has a guidetrack having an upper curved part and a lower linear part at each end ofthe curved part, each of the linear parts having a first length in thedirection of the projecting parallel portions, said first length foreach of the linear parts extending from a side of the piston rod guidefrom which the respective portion projects to an end of the curved part,a second length taken from said side of the piston rod guide to thecenter of the piston rod at the top of the curved part, said secondlength being equal to the sum of the length of one of the linear partsplus the radius of the curved part, said second length being equal to orlarger than the distance between the axes of the two extending parallelportions, wherein the piston rod is formed by a flexible helix withnarrowly adjacent turns of windings, the flexible helix having a coilingratio

    r.sub.coil = d.sub.helix-d.sub.wire !÷d.sub.wire <5.0,

where d_(helix) is an outer diameter of the wound helix, and d_(wire) isa diameter of the wire.
 2. A displacement system according to claim 1,wherein the curved part of the guiding track has in the shape that thecurved part of the piston rod will spontaneously adopt when its endportions are kept parallel.
 3. A displacement system according to claim1, wherein r_(coil) <4.5.
 4. A displacement system according to claim 3,wherein r_(coil) <4.0.
 5. A displacement system according to claim 4,wherein r_(coil) <3.5.
 6. A displacement system according to claim 1,wherein the windings of the helix provide an external thread.
 7. Adisplacement system according to claim 6, further comprising a pistonrod drive for transmitting a drive force to the piston rod through a nutelement having an internal thread engaging the external thread of thepiston rod.
 8. A displacement system according to claim 1, wherein thedriving force is transmitted to the piston rod at a linear part of therod.
 9. A displacement system according to claim 8, wherein the drivingforce is transmitted to the piston rod at a part thereof disposedbetween the rear end of the cartridge and the piston rod guide.
 10. Adisplacement system according to claim 1, wherein a driving force isexerted on a free, deflected end of the piston rod in the axialdirection.
 11. A displacement system according to claim 1, wherein saidlength for each of the linear parts are substantially equal.